Scott's Gif Fixer

http://www.sb-software.com/giffix

Purpose:

This program is used to fix the gif screen descriptor in Gif 87a and (some) Gif 89a images. I wrote this program because I had several gif images that did not resize properly when displayed on web sites. The problem turned out to be the "screen size" that is encoded into the gif images. These (very old) images had been created with very old image editing software, which had places the screen resolution (in my case, 1152x864) in the "screen size" field rather than placing the image size (in my case 480x480) in the "screen size" field.

Technically, this should be fine ... and it's how GIF is supposed to work

However, there are a few buggy viewer programs out there that confuse the "screen size" and "image size" fields. These (buggy) viewers display the incorrect aspect ratio when they try to resize the image.

Technical Details:

GIF files have two different resolutions stored in them:

  1. The "screen resolution", which is stored in the global header of the Gif file
  2. The "image resolution", which is stored in the image header of the Gif file

The way the Gif file format is intended to work is that the "screen resolution" should be set to the resolution of the computer where you designed the image, and the "image resolution" is set to the actual resolution of the image.

At some point, the conventions changed, and most image programs started placing the "image resolution" in both of these fields, and this is a perfectly acceptable thing to do.

Some "viewers" also make the same assumption -- that these two fields are the same. Now, the problem occurs if your GIF creation software places the screen resolution in the "screen resolution" field (as it should), and the GIF viewer software assumes that the screen resolution is the same as the image resolution. At this point, the viewer has made an incorrect assumption, and displays the image with an improper aspect ratio.

Using the program:

The program has on-screen instructions and it should be very easy to use.

Any questions or comments -- send them to smbaker@sb-software.com